Sinclair ZX80 / ZX81 / Z88 Forums

I was hoping someone could assist me.

I built a ZX80 kit which follows the original 1980 track layout. The machine produces a raster complete with little distortions every few pixels and a little kink at the bottom. Exactly as my other working board does, except the "K" prompt is missing.

I have tested the board with a known good processor and ROM (2532 with Vpp tied to Vcc). I even made a 27C32 to ZX80 Mask ROM adapter which requires the swapping of the two lines, A11 and E etc.

I'm also using the exact same clock crystal configuration as my other board.

Could you please give me an idea of where to look next?

Could please you post a photo of the screen display that you are getting.

Mark

I'll upload a picture, but there's not much to see. It looks EXACTLY like it does when the K is present (along with all the normal background image artefacts), there's just no "K".

http://www.myprius.co.za/zx80error.zip

Sorry for the ZIP file, but I wanted to get as high rez as possible. Two photos of the early build snuck in.

Thank you for trying to help.

I would say CPU or RAM. Do You have another Z80 to swap?

I plugged in a known working RAM pack (which disables onboard RAM) and also swapped in a known working CPU to no avail.

What is the Z80 doing?

If you have suitable test equipment, test the clock input on pin 6, and the various control output signals. Also the data bus and at least address pins A0, A1, A14 and A15.

Mark

C11 was missing! I forgot to put in one tiny 47pF capacitor. I'm such an idiot!

It gives a "K" prompt now. Thanks, everybody for trying to help.

The ZX is not so forgiving. Everything has to be in order for the picture to show, because the picture is purely software

albertusj wrote: ↑Tue Dec 10, 2019 10:59 am C11 was missing! I forgot to put in one tiny 47pF capacitor. I'm such an idiot!

I also won't be happy, if my C5 would be missing ...

forum.tlienhard.com/phpBB3/viewforum.php?f=10



Siggi